Growth, Differentiation and Sexuality

domains of the HD1 proteins are predicted to
contain coiled-coil α-helices that mediate protein
dimerisation in other transcription factors. Two
such domains were predicted in the C. cinereus
proteins and, significantly, the relative positions
of these were different in proteins coded by
paralogous a, b and d genes (Banham et al. 1995).
For the U. maydis proteins, it was shown that
single amino acid substitutions were sufficient
to convert a normally incompatible protein pair
into a pair that could dimerise and, significantly,
these substitutions caused either an increase in
hydrophobicity or a change in charge, both consistent
with changes affecting coiled-coil interactions
(Kämper et al. 1995).
Heterodimerisation plays an important role in
regulating transcription factor function. Studies on
DNA binding by the S. cerevisiae a1/α2 heterodimer
illustrated the importance of dimerisation in determining
DNA-binding specificity (Johnson 1995).
In C. cinereus, ithasbeenshownthattheHD1
protein provides the likely activation domain and
the nuclear targeting sequences, but has a dispensable
DNA-binding domain whereas the HD2 protein
provides the essential DNA-binding domain
(Asante-Owusu et al. 1996; Spit et al. 1998). Here,
separation of functional domains into two proteins
represents an elegant strategy to ensure that
mating-dependent developmental pathways are activated
only after fusionbetweencompatible mates.
3. The a and B Genes Encode Pheromones
and Receptors
Pheromone signalling plays an essential role in
mating in both ascomycete and basidiomycete
fungi (see also Chap. 16, this volume) but it is
only in the latter that the genes have become
mating type determinants, and only in homobasdiomycetes
are these genes multiallelic. The two
alleles of the a locus of U. maydis, firstdescribed
by Bölker et al. (1992), consist of very dissimilar
DNA sequences bordered by regions of homology,
and each contains two genes that encode a mating
type-specific pheromone and the corresponding
receptor. The a1 locus spans 4.5 kb whereas the
a2 locus spans more than 8.0 kb.Thepheromones
areunabletoactivatethereceptorswithwhich
they are found, and the dissimilarity in sequence
throughout the two different versions of this
locus ensures that recombination cannot generate
compatible receptor–pheromone combinations.
(There are two additional genes, lga1 and rga1, in
Mating Type Genes in Basidiomycetes 365
the a2 locus that encode mitochondrial functions
activated by mating but not relevant to it; Bölker
et al. 1992; Bortfeld et al. 2004.)
In C. cinereus, the corresponding B locus spans
some 17 kb and is far more complex than the U.
maydis a locus (O’Shea et al. 1998; Halsall et al.
2000). As at the A locus, we find three tandemly arranged
groups of genes that are functionally redundant.
These have been designated groups 1, 2 and 3.
In the B42 locus illustrated in Fig. 17.5, each group
comprises a receptor gene and two pheromone
genes but, in other loci, pheromone gene numbers
range from 1 to 3 and the orders of the genes and
their orientation are variable (Riquelme et al. 2005).
As with the A genes, locus integrity is maintained
by the dissimilarity in sequence between allelic versions
of the genes and the flanking sequence in
which they are embedded, so that recombination
cannot bring together compatible gene combinations.
Mating partners are compatible if they bring
together different alleles of just one group of genes,
these genes encoding a compatible complement of
pheromones and receptors (O’Shea et al. 1998). As
with the A locus, evidence points to large numbers
of B specificities being generated by the different
allele combinations of these three sets of genes.
Molecular characterisation of 13 B specificities in
C. cinereus has identified sufficient alleles to generate
70 unique combinations and, hence, different B
specificities, close to the 79 predicted by population
studies (Riquelme et al. 2005). It is not clear why
there are so many pheromone genes in C. cinereus;
few show any difference in specificity, and though
they cannot activate a self-receptor, most appear to
activate all other receptors within the same group,
i.e. group 1 pheromones activate group 1 receptors,
group 2 pheromones activate group 2 receptors
and group 3 pheromones activate group 3 receptors
(Riquelme et al. 2005).
In S. commune, the pheromone and receptor
genes are separated into two groups that correspondtotheBα
and Bβ loci identified in classical
recombination analyses, when it was established
that there are nine alleles of each locus (Raper et al.
1958). Bα and Bβ have each been shown to contain
a receptor gene (bar and bbr)andavariablenumber
of pheromone genes (bap and bbp; Wendland
et al. 1995; Vaillancourt et al. 1997; Fowler et al.
2004). Not all crosses between Bα and Bβ specificities,
however, yielded recombinants (Stamberg
and Koltin 1971). Molecular analysis of the Bα3–
Bβ2 complexillustratedinFig.17.5providedthe
answer to this puzzle and a surprising twist to our